The invention herein relates to a method for controlling the density of expanded perlite.
Perlite is a volcanic rock which contains small amounts of water within its structure. When perlite granules are rapidly heated to temperatures on the order of about 1800.degree. F. the water turns to steam and the perlite granules "pop" or expand to a volume of 4 to 20 times the volume of the unexpanded granules, with an attendant decrease in the bulk (or "apparent") density of the perlite. In this expanded form the perlite has found many such uses, such as lightweight aggregate and as a noncombustible lightweight thermal insulation.
In many of the perlite use applications (such as insulating boards) substantial uniformity of perlite bulk density is important in order to obtain optimum end products. Consequently, it is imperative for the expanded perlite manufacturer to control closely the expansion process and to act promptly to correct any deviations in the product from the desired final density properties.
In the past it has been common practice to determine the bulk density of the perlite product by taking periodic samples from the collection bin where the expanded perlite is deposited. The density of these samples was determined by conventional density measurements. The results of these tests were then reported back to the expander operators and the operation of the expanders adjusted accordingly where deviations from the desired density were discovered. This procedure is obviously very time consuming and results in significant delays in the discovery of processing irregularities. This in turn can lead to the production of large quantities of unusable product during the period between testing intervals.
It would therefore be advantageous to have a system which would monitor the density of the expanded perlite on a substantially continuous basis. It would also be very advantageous to have a system in which the monitoring device would also generate appropriate signals to related equipment which would promptly alter the operation of the system so that the desired density could again be obtained while yet minimizing the amount of "off spec" material which would be produced before the system controls could be corrected.